Natural convection in semi annulus enclosures in presence of magnetic field are investigated numerically. Lattice Boltzmann method (LBM) is applied to simulate these problems. Two useful models are presented in order to simulate magnetic field. The working fluid is nanofluid. Different models for simulating nanofluid hydrothermal behavior are presented. The results reveal that average Nusselt number is an increasing function of nanoparticle volume fraction as well as Rayleigh number, while it is a decreasing function of Hartmann number.